Adding Blender Datapaths together as strings and then executing the resulting string
You can use 

eval(b)


eval() will take any valid Python expression string as argument.

To set a value, use

exec(c)


But be careful, exec() will execute ANY python code, so it is considered a very dangerous command, especially if the string is from user input.

I would advice rethinking how your code is setup, there is usually very little reason to access bpy in these fashion.